THE NERVOUS SYSTEM J7:<
the later work of Shemngton a::d C4runb?ji!:* 80, ,.>:. the Orang, Chimpanzee and Groriifo. The latter ob>er\vr-showed that the motor centres are localized la tl:e pro-central gyrus and are continued into the cortex v»h:e".i dips into the central ^ulcus, and the secondary suL-i I.M which this is limited. As the cortex loses its instability the anterior boundary of the motor area retreat-* lack-wards, but the posterior limit always remains fixed. Xu motor centres exist in the postcentral gyrus ind the island of Rei! is inexcitable. In the frontal cortex anterior to the Re Ian die area there is tin area which. when stimulated, produces conjugate deviation o: the eyes, an action which is also elicited when the cortex round the caicarine fissure is stimulated. This Matter region, however, is not part of the motor area, which lies only in the frontal lobe.
The parietal and occipital lobes are separated by the parieto-occipital sulcus which, however, is very variable in character in each species of Ape as in Man. It may be entire and deep, or it may be replaced by small sulci, thus showing that it is a complex when it is entire. A deep groove may have two sulci at the bottom.
The parietal and temporal lobes are separated by the fissure of Sylvius, whose characters differ considerably. It may be stated at the outset that Man alone of the Primates has a complete Sylvian fissure. It consists in him of three fissures—an anterior horizontal, an anterior ascending, and a posterior horizontal limb. These fissures run between undercut pieces of cerebral tissue, which constitute the opercula covering and concealing entirely the island of Reil. In the Apes the conditions are somewhat different. The posterior horizontal limb